A loudspeaker typically has a voice coil comprising a conductor through which a current may be passed, placed within a magnet assembly so that when current is passed through the voice coil an electromagnetic driving force is produced. This in turn drives a driven body, such as a diaphragm. Conventionally, this vibrates along the loudspeaker axis (i.e. the axis which passes from a front to a rear of the loudspeaker and which is substantially central to the loudspeaker, around which the loudspeaker is usually substantially rotationally symmetric), driven by a driver mechanism such as a voice coil as described. The movement of the diaphragm creates a pressure wave in the surrounding air, which propagates as a sound wave.
A loudspeaker diaphragm inevitably resonates at certain frequencies, the lowest of which is described in the art as a “breakup” frequency. Such resonances are generally undesirable, as they involve movements of the diaphragm that do not correspond to the electrical signal that is being applied via the voice coil. Thus, if they fall within the frequency range of the loudspeaker (or close to it) then they will affect the sound output. Generally, a stiff diaphragm will show resonance at a higher frequency, and efforts are often directed at increasing the stiffness of the diaphragm so that the resonant frequency is pushed to a higher level, allowing a higher frequency range for the loudspeaker. To this end, a concave cone diaphragm is commonly used as this combines a good surface area with an inherently stiff shape. A suspension is often attached to a rear face of the cone, and comprises a structure with a gentle elastic response in order to provide a restoring force to the diaphragm.
The diaphragm is typically set within an elastically-deformable surround which encircles the diaphragm and connects its outer edge to a fixed support structure. This provides a further gentle restoring force, and also creates an air seal separating the air volumes in front of and behind the diaphragm. The latter prevents dissipation of the pressure wave created by movement of the diaphragm by air flow to or from the converse-pressure region behind the diaphragm.
Where a loudspeaker is designed with a view to minimising its overall depth, such as where the available depth is at a premium, a simple cone shape may be impractical as it lends a significant depth to the loudspeaker. However, it remains important to maintain the stiffness of the diaphragm. Various options exist to cope with this challenge. One such method is shown in US2010/0208934 and US2012/0106776, and involves forming the diaphragm as a double cone, having a radially inner convex cone shape (as viewed from the exterior of the loudspeaker) and a radially outer concave cone shape. Viewed in section, the diaphragm thus adopts a “V” profile. The suspension is attached to the diaphragm at the apex of the V shape and the inner and outer edges of the V are attached to the voice coil and the surround respectively. Another solution is shown in our application GB2479941A, which describes a generally flat loudspeaker diaphragm with stiffening ribs projecting transversely away from a surface of the diaphragm.
A key requirement of a loudspeaker with a shallow overall depth is the provision of a sufficient excursion capability, so that a sufficient volume of air can be moved by the loudspeaker to meet the requirements thereof. This is especially relevant when the loudspeaker is intended to be used to generate sound in the lower-frequency ranges.
It is also a challenge that a simple flat shallow diaphragm may suffer from “rocking” movements, i.e. rotational oscillations about an axis perpendicular to the loudspeaker axis. Rocking vibrations may become problematic if the suspension of the loudspeaker is arranged in a plane that is close to the plane of the surround, as the combined moment that can be exerted by the two restoring forces to counteract a rocking movement is small. A rocking movement can cause the voice coil to become misaligned within the magnet arrangement and may cause contact between them, leading to damage to the voice coil.